Process for the synthesis of alkylated aromatic hydrocarbons

ABSTRACT

Process for the synthesis of alkylated aromatic hydrocarbons containing a saturated alkyl chain comprising at least four carbon atoms, according to which an aromatic hydrocarbon substituted by a saturated short-chain alkyl group comprising one to three carbon atoms is reacted with an olefin in the presence of a catalyst which comprises at least one alkali metal or one alkali metal hydride impregnated on an alumina support and the catalyst is prepared in the reaction medium in the presence of the aromatic hydrocarbon containing a short alkyl chain by mixing anhydrous alumina with the alkali metal hydride or with the alkali metal, the ratio by weight of the alkali metal or of the alkali metal hydride to the alumina support being between 0.6 and 1.8. Use of the process for the synthesis of tert-amylbenzene by alkylation of cumene with ethylene.

CROSS-REFERENCE TO RELATED APPLICATION

The present application is a continuation-in-part of application No.07/768,988 filed Jan. 24, 1992, now U.S. Pat. No. 5,288,936.

FIELD OF THE INVENTION

The present invention relates to a process for the manufacture ofalkylated aromatic hydrocarbons. More particularly, it relates to Themanufacture of alkylated aromatic hydrocarbons containing an alkyl chaincomprising at least four carbon atoms by condensation of an aromatichydrocarbon, substituted by a short-chain alkyl group containing 1 to 3carbon atoms, with an olefin in the presence of a catalyst.

TECHNOLOGY REVIEW

Application FR-A-2,647,438 (INTEROX) discloses a process for thesynthesis of alkylated aromatic hydrocarbons which consists in reactingan aromatic hydrocarbon, substituted by a short-chain alkyl group, withan olefin in the presence of a catalyst prepared in the reaction mediumby mixing anhydrous alumina with an alkali metal or an alkali metalhydride. The alkali metal/alumina or alkali metal hydride/alumina ratioby weight is preferably between 0.02 and 0.5.

It has been surprisingly found that it was possible to greatly increasethe productivity of the reaction if the alkali metal/alumina or alkalimetal hydride/alumina ratio by weight was adjusted so that it lies in arange of higher values.

SUMMARY OF THE INVENTION

To this end, the invention relates to a process for the synthesis ofalkylated aromatic hydrocarbons containing a saturated alkyl chaincomprising at least four carbon atoms, according to which an aromatichydrocarbon substituted by a saturated short-chain alkyl groupcontaining one to three carbon atoms is reacted with an olefin in thepresence of a catalyst which comprises at least one alkali metal or onealkali metal hydride impregnated on an alumina support, the catalystbeing prepared in the reaction medium in the presence of the aromatichydrocarbon containing a short alkyl chain by mixing anhydrous aluminawith the alkali metal or with the alkali metal hydride; according to theinvention, the ratio by weight of the alkali metal or of the alkalimetal hydride to the alumina support is between 0.6 and 1.8.

DETAILED DESCRIPTION OF THE INVENTION

The invention is targeted at preparing aromatic hydrocarbons alkylatedby a saturated, linear or branched alkyl chain comprising more than fourand, preferably, not more than eight carbon atoms. It applies inparticular to the preparation of benzene hydrocarbons alkylated by asaturated and branched alkyl chain comprising five carbon atoms. It isvery particularly suited to the preparation of amylbenzenes such as, forexample, tert-amylbenzene.

Alkylated aromatic hydrocarbon containing a short chain used as startingmaterial according to the invention is understood to denote any aromatichydrocarbon containing one or a number of aromatic rings containing fiveor six carbon atoms and at least one saturated alkyl side chaincomprising one to three carbon atoms. Benzene hydrocarbons containing asingle aromatic ring are preferred and very particularly toluene,dimethyl and trimethylbenzenes, ethylbenzene, ethyltoluenes,n-propylbenzene and cumene.

The olefin used as alkylating agent must contain a linear carbon chaincomprising at least two and not more than five carbon atoms, at leasttwo of them being bonded by an unsaturated olefin-type bond. Monoolefinswhich only have a single unsaturation are preferred. By way of examples,the following olefins can be used in the process according to theinvention: ethylene, propylene, n-butenes, isobutene, butadiene,n-pentenes and methylbutenes.

The catalyst used in the process according to the invention must containat least one alkali metal or one alkali metal hydride in addition to thealumina support. Any alkali metal may be suitable. Sodium or sodiumhydride are, however, generally preferred due to their good activity andtheir ready availability. Sodium can be the only alkali metal presentwith the alumina support; such a catalyst has given excellent results.

A variant of the catalyst in accordance with the process according tothe invention consists in using an alumina support and a mixture ofalkali metals or of alkali metal hydrides in substantially equalproportions or, in another variant, such that a single one of the alkalimetals or of the alkali metal hydrides does not exceed double theproportion of all the other combined alkali metals or alkali metalhydrides.

Another particularly advantageous variant of the catalyst in accordancewith the process according to the invention consists in mixing, with amain alkali metal or with an alkali metal hydride, small amounts, notexceeding a few percent, of one or of two other alkali metals or alkalimetal hydrides as promoters and of adding them to the alumina support.The mixture of sodium or of sodium hydride with more than 0.1% ofrubidium and/or of caesium or of rubidium and/or caesium hydride is veryparticularly worthy of interest. Likewise, it is advantageous that themixture of sodium or of sodium hydride with rubidium and/or caesium orrubidium hydride and/or caesium hydride contains less than 5% ofrubidium and/or of caesium or less than 5% of rubidium hydride and/or ofcaesium hydride.

It is also possible to combine, in the same catalyst, a mixture ofalkali metal with an alkali metal hydride.

The amounts of alkali metal or alkali metal hydride used are preferablysuch that the catalyst has an alkali metal/alumina or alkali metalhydride/alumina ratio by weight greater than 0.7. It is alsoadvantageous that this ratio by weight is less than 1.5.

According to the invention, the alumina support can consist of aluminaof different, pure or mixed, crystalline varieties. The α-, β- andγ-alumina varieties are well suited. Good results were obtained with asupport consisting solely of γ-alumina.

The aluminas used as catalyst support in the process according to theinvention are porous substances. They generally have a mean porediameter greater than 0.1 nm. Likewise, this mean pore diameter is oftenless than 500 nm. Advantageous results were obtained with aluminas inwhich the mean pore diameter was greater than 2 nm. Good results werealso obtained with aluminas in which the mean pore diameter was lessthan 100 nm. The best results were obtained by using aluminas with amean pore diameter of 20 and 30 nm.

In order for the catalyst obtained by mixing an alkali metal or analkali metal hydride with these aluminas to be efficient, the specificsurface of these aluminas must generally be greater than 10 m² /g. It isalso advisable that the specific surface of these aluminas is less than360 m² /g. In practice, aluminas are preferred in which the specificsurface is greater than 50 m² /g. Likewise, aluminas in which thespecific surface is less than 200 m² /g are preferred.

The aluminas which are suitable for accompanying the alkali metals inthe catalysts in accordance with the process according to the inventiongenerally have a pore volume greater than 25 ml/100 g and preferablygreater than 45 ml/100 g. The pore volume of these aluminas is mostoften less than 75 ml/100 g and preferably less than 65 ml/100 g.

The aluminas used must be free of traces of free water. If necessary, itcan be useful, prior to their use as alkali metal support, to calcinethem at a temperature greater than 200° C. with the aim of removing anytrace of residual moisture. The calcination temperature of thesealuminas most often does not exceed 600° C.

The optimum amounts of catalyst are not critical. However, it isadvisable, to ensure that the catalyst has a good efficiency, that theratio by weight of the alkali metal to the short-chain aromatichydrocarbon is greater than 0.0015. Likewise, it is advantageous thatthis ratio by weight remains below 0.03.

An advantageous variant of the process according to the inventionconsists in adding a promoter such as potassium hydroxide during thepreparation of the catalyst in the reaction medium. Potassium hydroxideis generally used in an amount such that the K hydroxide/alkali metal oralkali metal hydride ratio by weight is equal to or greater than 0.3 andpreferably equal to or greater than 0.5. This ratio by weight is alsooften less than 2 and preferably than 1.5.

The temperature and the pressure at which the alkylation reaction inaccordance with the process according to the invention is carried outmust be suited to the nature of the starting aromatic hydrocarbon andolefin. However, it was observed that pressures greater than 0.5 MPa andtemperatures greater than 110° C. are well suited. Likewise, it wasobserved that appropriate pressures are generally less than 10 MPa andthat the temperatures are often below 250° C. Pressures greater than 1MPa and temperatures greater than 170° C. have given the best results.The latter are also obtained with pressures below 5 MPa and temperatureslower than 200 C. and preferably lower than or equal to 198° C. When thereaction is carried out in the vapour phase, it is advantageous to setthe respective pressures of the olefin and starting aromatic hydrocarbonso that the olefin/aromatic hydrocarbon molar ratio is greater than 0.7.Likewise, in this case, it is also advantageous that this molar ratio isless than 1.3. An olefin/aromatic hydrocarbon molar ratio in the regionof 1 will preferably be chosen.

In order to ensure good contact between the reactants, it is generallynecessary to carry out the reaction with vigorous stirring, inparticular when the starting hydrocarbon is liquid and the olefin isgaseous under the reaction conditions. All known types of stirrers aregenerally well suited. The choice of a specific stirrer is madeaccording to the type of equipment used for the reactor.

The reaction in accordance with the process according to the inventioncan be carried out without distinction in a continuous or non-continuousreactor. When the reaction uses a system containing two differentphases, the continuous reactors can be chosen from conventional reactorssuch as stationary-bed reactors, fluidized beds or mobile-bed reactorswhich make it possible for the catalyst to move and optionally to beregenerated. If three phases are simultaneously present in the reactionsystem, it is possible to use conventional continuous stationary-bed(trickle bed) or slurry reactors. In the case of a non-continuousreactor, an autoclave equipped with a paddle stirrer is generally used.

The process according to the invention is particularly well suited tothe synthesis of tert-amylbenzene by alkylation of cumene with ethylene.

The examples which follow have the aim of illustrating the inventionwithout limiting the scope thereof.

EXAMPLES 1R and 2R (not in accordance with the invention)

200 ml of cumene, 1 g of sodium hydride, 0.5 g of KOH and a certainamount of alumina were introduced into an autoclave with a capacity of400 ml equipped with a paddle stirrer.

The autoclave was then closed and heated at 170° C. for 15 minutes withstirring and under a nitrogen purge. The reaction was then initiated byinjecting ethylene at a pressure of 1 MPa into the reactor.

The reaction was left to continue for one hour, after which thetert-amylbenzene formed and the residual cumene were quantitativelydetermined. The results were recorded in the following table.

    ______________________________________                                                           Degree of                                                  No. of NaH/Al.sub.2 O.sub.3                                                                      conversion                                                 the    ratio by    of the cumene,                                                                            Productivity                                   example                                                                              weight      %           g TAB/h · g cat.                      ______________________________________                                        1R     0.5         99.0        58.2                                           2R     2.0         0.0         0.0                                            ______________________________________                                    

In this table, the degree of conversion of the cumene is equal to 100times the molar ratio of the cumene consumed to the cumene used and theproductivity is equal to the weight in g of tert-amylbenzene (TAB)formed per hour and per g of catalyst.

EXAMPLES 3 to 5 (according to the invention)

Example 1 was reproduced while varying the weight of alumina.

The results obtained are given in the following table:

    ______________________________________                                                           Degree of                                                  No. of NaH/Al.sub.2 O.sub.3                                                                      conversion                                                 the    ratio by    of the cumene,                                                                            Productivity                                   example                                                                              weight      %           g TAB/h · g cat.                      ______________________________________                                        3      0.6         99.5        64.5                                           4      0.8         98.7        74.5                                           5      1.0         98.7        81.5                                           ______________________________________                                    

What is claimed is:
 1. A process for the synthesis of an alkylatedaromatic hydrocarbon containing a saturated alkyl chain comprising atleast four carbon atoms, comprising:reacting an aromatic hydrocarbonsubstituted by a saturated short-chain alkyl group containing one tothree carbon atoms with an olefin in the presence of a catalyst of atleast one alkali metal or one alkali metal hydride impregnated on analumina support, said alkali metal or alkali metal hydride having aweight ratio to said alumina support between 0.6 an 1.8, said catalystbeing prepared in the reaction medium in the presence of the aromatichydrocarbon containing a short alkyl chain by mixing anhydrous aluminawith the alkali metal or with the alkali metal hydride, and, recoveringan alkylated aromatic hydrocarbon containing a saturated alkyl chaincomprising at least four carbon atoms.
 2. The process according to claim1, wherein the alumina support has a mean pore diameter of between 2 and100 nm.
 3. The process according to claim 1, wherein the alumina supportconsists of γ-alumina.
 4. The process according to claim 1, wherein thealumina support has a pore volume of between 25 and 75 ml/100 g ofalumina and a specific surface area of between 10 and 360 m² /g ofalumina.
 5. The process according to claim 1, wherein the ratio byweight of the alkali metal or of the alkali metal hydride to theshort-chain aromatic hydrocarbon lies in the range of from 0.0015 to0.03.
 6. The process according to claim 1, wherein the alkali metal issodium and the alkali metal hydride is sodium hydride.
 7. The processaccording to claim 1, wherein the reaction is carried out at atemperature between 170° and 200° C.
 8. A process according to claim 1wherein said aromatic hydrocarbon is cumene and said olefin is ethylene.9. A process according to claim 2 wherein said aromatic hydrocarbon iscumene and said olefin is ethylene.
 10. A process according to claim 3wherein said aromatic hydrocarbon is cumene and said olefin is ethylene.11. A process according to claim 4 wherein said aromatic hydrocarbon iscumene and said olefin is ethylene.
 12. A process according to claim 5wherein said aromatic hydrocarbon is cumene and said olefin is ethylene.13. A process according to claim 6 wherein said aromatic hydrocarbon iscumene and said olefin is ethylene.
 14. A process according to claim 7wherein said aromatic hydrocarbon is cumene and said olefin is ethylene.